In semiconductor devices having solid state imaging devices such as a CCD (Charge Coupled Device) and a CMOS (Complementary Metal Oxide Semiconductor) to be used in digital still cameras and the like, optical glasses such as a near-infrared cut filter glass and a cover glass are used. In recent years, due to demands for a reduction in thickness of a solid state imaging device module to be mounted on a portable terminal such as a mobile phone or a smart phone and a reduction in thickness of a digital still camera, an optical glass having a thin plate thickness is required.
However, when the plate thickness of the optical glass is thinned, it becomes increasingly likely that fracture progresses starting from a chip or a microcrack existing in an edge line of the glass (boundary between a principal surface and a side surface of the glass) to cause breakage of the glass in the case when a bending stress acts on the optical glass.
Therefore, from a viewpoint of improving a bending strength of a glass, chamfering a glass edge surface has been proposed. This aims at increasing a bending strength of a glass by removing flaws in its glass edge surface to be a starting point of fracture by chamfering. Further, removing flaws in a principal surface of a glass plate by etching has been also proposed.
However, operations of the chamfering of a glass edge surface and the removing of flaws in a glass principal surface deteriorate (decrease) productivity of the optical glass. Further, a flaw is sometimes rather formed in the glass edge surface by the chamfering. This is caused because the chamfering of a glass is to mechanically process a glass with a grinding wheel. That is, an unintended flaw may be newly formed due to impact or the like during the chamfering. Further, when the etching is performed while holding the principal surface of the glass for the purpose of removing flaws in the principal surface of the glass, etching unevenness occurs on the principal surface being an optically effective surface, resulting in that optical characteristics as the optical glass may deteriorate (decrease).
In the meantime, as a cutting method of a semiconductor substrate and the like, Stealth Dicing (registered trademark) has been know. This cutting method is a technique of cutting a semiconductor element in a manner that laser light with a wavelength passing through a semiconductor substrate (for example, silicon (Si)) is collected inside the semiconductor substrate to form a reformed region (flaw region) inside the semiconductor substrate, and then an external stress such as a tape expansion is applied to cause a crack in the semiconductor substrate starting from the reformed region.
This cutting method enables the reformed region to be locally and selectively formed inside the semiconductor substrate without damaging the principal surface of the semiconductor substrate, and therefore it is possible to reduce occurrence of defects such as chipping in the principal surface of the semiconductor substrate that is a problem in general blade dicing. In addition, there are fewer problems such as dust occurrence unlike machining. Therefore, in recent years, the cutting method becomes to be widely used not only in cutting the semiconductor substrate but also in cutting a glass substrate.